refactor: update placeholder block and import details page

[navinkarkera]

Description

• Updates placeholder block color and icon.
• Moves View Imported Content & Retry import buttons in import details page inside alert at the top.
• Updates page title to include Import status in import details page.

Useful information to include:

• Which user roles will this change impact? "Course Author"

Supporting information

• Import Details Page #2526 (comment)
• Placeholder for failed blocks? #2527 (comment)

Testing instructions

• Check page title in import details page in library.
• Check action button position.
• Check placeholder block color and icon in library home page.

Other information

Include anything else that will help reviewers and consumers understand the change.

• Does this change depend on other changes elsewhere?
• Any special concerns or limitations? For example: deprecations, migrations, security, or accessibility.

Best Practices Checklist

We're trying to move away from some deprecated patterns in this codebase. Please
check if your PR meets these recommendations before asking for a review:

• Any new files are using TypeScript (.ts, .tsx).
• Avoid propTypes and defaultProps in any new or modified code.
• Tests should use the helpers in src/testUtils.tsx (specifically initializeMocks)
• Do not add new fields to the Redux state/store. Use React Context to share state among multiple components.
• Use React Query to load data from REST APIs. See any apiHooks.ts in this repo for examples.
• All new i18n messages in messages.ts files have a description for translators to use.
• Avoid using ../ in import paths. To import from parent folders, use @src, e.g. import { initializeMocks } from '@src/testUtils'; instead of from '../../../../testUtils'

[openedx-webhooks]

Thanks for the pull request, @navinkarkera!

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.90%. Comparing base (b7dd670) to head (e94188e).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #2761    +/-   ##
========================================
  Coverage   94.90%   94.90%            
========================================
  Files        1235     1235            
  Lines       28260    28284    +24     
  Branches     6433     6233   -200     
========================================
+ Hits        26819    26842    +23     
- Misses       1370     1384    +14     
+ Partials       71       58    -13     

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[ChrisChV]

Looks good 👍

• I tested this: I followed the testing instructions
• I read through the code and considered the security, stability and performance implications of the changes.
• I tested that the UI can be used with a keyboard only (tab order, keyboard controls).
• Includes tests for bugfixes and/or features added.
• Includes documentation